IPv6 Address Planning

Chapter 4. IPv6 Subnetting

As I was going to St. Ives,

I met a man with seven wives,

Each wife had seven sacks,

Each sack had seven cats,

Each cat had seven kits:

Kits, cats, sacks, and wives,

How many were there going to St. Ives?

— Traditional Nursery Rhyme

Introduction

We’ve discussed how early efforts to successfully slow the depletion of IPv4 included techniques like VLSM, CIDR, and NAT. In particular, the granular subnetting provided by VLSM became a common (and engrained) practice in IPv4 network architecture and address planning. But the enormous scale of IPv6 and the resulting bounty of additional bits in a given address require new subnetting methods. These methods provide opportunities to improve both the ease and effectiveness of IPv6 address planning. In this chapter, we’ll cover these IPv6 subnetting techniques and the legacy IPv4 subnetting methods they differ from (and improve upon).

Subnetting IPv4: A Brief Review

Before we dig into IPv6 subnetting methods, let’s briefly review their counterparts in IPv4.

As we’ve already discussed, subnetting in IPv4 optimistically started out as class-based; i.e., using only two classes of subnets to facilitate aggregation and reduce the demand placed on router memory and CPU resources (as well as create some hierarchical consistency within the Internet).

The first two classes of subnets in IPv4 were as follows:

Class A

8 bits to identify the network, 24 bits for host addressing

Class B

16 bits to identify the network, 16 bits for host ...